The present invention relates to a tape- or sheet-formed electrical insulating material comprising an insulating layer in the form of a homogeneous film of an organic polymer or in the form of a felt or a paper of fibres of an organic polymer.
Films of organic polymers and felts of fibres of organic polymers generally have a very high electrical strength. In many cases, however, their usefulness as electrical insulating materials is limited by a high sensitivity to corona (partial discharges).
According to the present invention it has been found to be possible to protect such films and felts against corona damage so that they can withstand corona for a considerably longer period of time than previously. According to the invention, this is achieved by arranging on one or both sides of the film or felt a layer of an organic polymer containing a sufficient quantity of at least one of the substances chromium oxide and iron oxide in powder form.
One possible explanation of the favourable results obtained according to the present invention may be that the corona sputters away insulating polymer materials from the mentioned layer and that the chromium oxide or iron oxide thus exposed, because of its lower resistivity, increases the electrical conductivity locally on the surface sufficiently for the extremely concentrated effect of a corona discharge to spread over a larger surface and hence be effectively reduced.
More particularly, the present invention relates to a tape-formed or sheet-formed electrical insulating material comprising an insulating layer in the form of a homogeneous film of an organic polymer, or in the form of a felt or a paper of fibres of an organic polymer, which is characterized in that the insulating layer, at least on one side thereof, is coated with a protective layer which protects the insulating layer against a degradation caused by corona and which comprises an organic polymer containing at least 10 percent by volume of a powdered filler in the form of chromium oxide (Cr.sub.2 O.sub.3), iron oxide (Fe.sub.2 O.sub.3) or a mixture of chromium oxide (Cr.sub.2 O.sub.3) and iron oxide (Fe.sub.2 O.sub.3). The powdered filler preferably has an intrinsic resistivity of 10.sup.4 -10.sup.8 ohm m.
The protective layer containing the filler or the protective layers together, if one protective layer is arranged on each side of the polymer film, preferably has/have a thickness which is smaller than the thickness of the insulating layer.
The protective layer preferably has a resistivity exceeding 10.sup.10 ohm m.
Chromium oxide is preferred as filler because of its property of giving the protective layer a corona resistance which is not--or only insignificantly--reduced by the influence of external factors, such as moisture. The particle size of the powdered filler is suitably 0.005-30 .mu.m, and preferably 0.005-5 .mu.m. The content of the powdered filler suitably amounts to 10-40% of the volume of the protective layer. Particularly preferred is a content of powdered filler of 10-30% of the volume of the protective layer.
The organic polymer in the insulating layer, if this consists of a polymer film, may, among other things, be of polyimide, polyamide, polyamideimide, polyethyleneglyclol terephthalate, polycarbonate, polysulphon, polypropylene and polymethyl pentene.
The organic polymer in the fibres in the insulating layer, if this consists of a felt or a paper, may, among other things, be of polyethyleneglycol terephthalate, of an aromatic polyamide, of an aliphatic polyamide, of polypropylene, or of cellulose. Such felts are usually kept together by the fibres being felted together without orientation and possibly calendered.
The organic polymer in the protective layer may, among other things, consist of a resin normally used for enamelling electrical conductors or for impregnating electrical windings such as terephthalic acid alkyds, polyesterimides, polyamideimides, polyimides, polyurethanes, epoxy resins, polysulphons, silicones, polyamides, polymers based on polyhydantoin and unsaturated polyester resins and which has been converted into solid state.
The thickness of the protective layer, or the total thickness of the protective layers when using two protective layers, suitably amounts to 5-100 .mu.m, preferably to 10-50 .mu.m, and the thickness of the insulating layer suitably to 10-500 .mu.m, and preferably to 10-200 .mu.m.
An electrical insulating material according to the present invention is suited, among other things, as an insulating material for insulation of electrical conductors, as an insulating material in insulations between winding and machine slots in electrical machines using form wound coils, as slot insulation between winding and machine slots in machines using random wound coils, and as phase insulation between coils for different phases in the latter machines.
The invention will be explained in greater detail by describing examples with reference to the accompanying drawings: